Migrate Zimbra

HI Friends,

in my organization we are using zimbra mail server for internal only.
now we are trying make a new single server (on Cent OS 5.5) which can work as a internal and as well as external mail server,
my problem is we are changing the zimbra doamin (ex: raghav.com) name is different from the old name (ex: ho.raghav.com).
so Please suggest me how to migrate? zimbra Accounts / Mail boxs / Ldap .. Please suggest the better steps to do.....

Both old and new servers are in CentOS. but domain names are different .....